In a method for measuring distances using ultrasound sensors, which respectively have an emitter for transmitting measuring pulses and a receptor for receiving measuring pulse echos, the distance from an obstacle that reflects the measuring pulse is determined from the time difference between the measuring pulse and the reflected measuring pulse echo. This method may be implemented using a measuring device including: a) at least two ultrasound sensors which each have an emitter for transmitting measuring pulses and a receptor for receiving measuring pulse echos; b) a control unit; and c) an evaluation unit connected to the distance sensors.
The corresponding measurements of distance may be used for parking-assistance functions in motor vehicles, e.g., in order to detect obstacles in the surroundings of the vehicle, such as in front of, or behind, the vehicle, and to indicate the existence of these obstacles optically and/or acoustically. For this purpose, the ultrasound sensors emit a measuring signal, and the running time is measured until the measuring signal echo reflected from the obstacle arrives again at the receptor of the distance sensor. The running time is then recalculated into a distance. The driver is informed about the distance between the vehicle and the obstacle, whereby several advantages may be derived, e.g., more convenient parking and maneuvering, utilization of tighter parking spaces even for unpracticed drivers, and the avoidance of parking accidents.
Conventionally, after an obstacle is detected, the measurement is repeated at an interval that is as short as possible, in order to verify the result. However, in this context, one needs to consider that the adjacent distance sensors to the transmitting distance sensor are also used as receptors, which means reflecting measuring pulse echos may be derived from a wider spatial region. The measuring pulse echos received are designated as direct echos if a distance sensor receives again a measuring pulse that it transmitted itself, i.e., by direct reflection. However, in addition, cross echos may be observed which are measuring pulses of immediately adjacent distance sensors received by a given distance sensor as a result of cross reflection. If several distance sensors emit pulses substantially simultaneously, cross feed echos also appear under certain circumstances, which are cross-reflected measuring pulses received by a distance sensor, and which originate from another distance sensor than the receiving distance sensor or from the distance sensors directly adjacent to the receiving distance sensor.
Now, in order to prevent mutual interferences of the distance sensors and to be able to match the received measuring pulse echos to individual emitters of the distance sensors, not all distance sensors may emit at the same time. In the case of the four to six distance sensors usually used per vehicle side, e.g., in the case of six distance sensors in the front region of the vehicle, the reaction time is increased disadvantageously by the sequential distance measurement of the individual distance sensors, i.e., the measurements are staggered in time. In an attempt to overcome this shortcoming, emitting groups may be used, within which group up to two distance sensors emit simultaneously. Between the two emitters of a given emitting group, there are three other distance sensors of other emitting groups, for example, in order to ensure the correct matching of the received measuring pulse echos.
In the case of greater measuring ranges, there is additionally the danger that interference, so-called cross feed, may appear within the emitting groups which are arranged for short measuring ranges. Therefore, it is necessary in conventional approaches to limit the measuring range or to emit the measuring pulses sequentially by the distance sensor, which has a negative influence on the reaction time of the measuring system.
In FIG. 4, a conventional method for providing parking assistance in motor vehicles is shown in detail. In FIG. 4, the front end 3 of a motor vehicle is shown with six distance sensors 41, 42, 43, 44, 45 and 46, which may be, e.g., built into the bumper of the motor vehicle. In the conventional method shown, distance sensors 4 are operated in parallel in emission groups, i.e., measuring pulses MI of the individual emission groups are emitted substantially simultaneously. The lines that have bidirectional arrows show, in this case, direct reflection echos D, i.e., measuring pulse echos ME, which are emitted by an emitter of a distance sensor 4 and received again by the receptor of the same distance sensor 4 after direct reflection at an obstacle 7.
On the other hand, the lines having arrows directed one way represent cross reflection echos K, which are received as measuring pulses MI of a first distance sensor 4 after reflection by obstacle 7 or, in an exceptional case, possibly also by direct cross feed from the receptor of another distance sensor 4.
In FIG. 4, it may be seen that the emission group including distance sensors 41 and 45 simultaneously emit measuring pulses MI. These distance sensors 41, 45 then measure measuring pulse echos ME as direct reflection echos D. Distance sensor 42, which is adjacent to emitting distance sensor 41, also receives a direct cross reflection echo K. In a similar manner, distance sensors 44 and 46, which are adjacent to emitting distance sensor 45, receive direct cross reflection echos K of measuring pulses MI emitted by distance sensor 45.
Since it is known that only distance sensors 41 and 45, which are far apart from each other, emit actively at the same time, it may be ensured that no cross feed between the two emitting distance sensors 41 and 45 takes place.
Subsequently, the emission group including distance sensors 42 and 46 is activated. Here too, the distance between the emitting distance sensors 42 and 46 is sufficiently great that cross feed does not take place.
For the subsequent activation of distance sensor 43, however, it cannot be ensured that there is a sufficient distance from another distance sensor 4 that could also possibly be activated at the same time. Therefore, distance sensor 43 is operated by itself. The same applies to the subsequent activation of distance sensor 44.
In FIG. 5, a conventional distance measurement using emission groups involving four distance sensors 41, 42, 43 and 44 is shown, along with the direct reflection echos D and the cross reflection echos K. On account of the distances of distance sensors 4 from one another, cross feed can not be completely excluded, so that distance sensors 41, 42, 43 and 44 are activated individually (in sequence) for the first measurement and the verification measurement, respectively. Consequently, in the case of the first measurement E and the verification measurement V, in each case only a single distance sensor 4 is emitting. This disadvantageously results in a relatively great reaction time.
If now, as shown in FIG. 6, distance sensors 4, which are relatively close to one another, simultaneously emit measuring pulses MI, then, in the case of a lateral obstacle 7, cross reflection echos K may come about, thereby causing potential misinterpretations.
Measurement pulse echos ME received by distance sensor 43 are conventionally interpreted as being assigned to directly adjacent emitting distance sensor 44. By doing this, a pseudo-obstacle 8 is detected which, in actuality, does not exist. Instead, distance sensor 43 has merely received a cross reflection echo K from distance sensor 41 that is also emitting, which, with reference to emitting distance sensor 44, represents a cross feed echo. Accordingly, it may be seen that the emitting groups may not lie too close together, if a reliable distance measurement (e.g., for parking assistance) is to be implemented.
FIGS. 7 and 8 illustrate situations in connection with the conventional distance-measuring method, in which cross feed takes place within emitting groups when the measuring range is too great. The actual obstacle 7 is correctly detected by emitting distance sensor 41 from direct reflection echo D. Based on the great measuring range, measuring pulse MI emitted by distance sensor 41 is reflected by obstacle 7 right up to distance sensor 45, as shown in FIG. 7, or to distance sensor 44, as shown in FIG. 8. Since, in addition, distance sensor 4. (in FIG. 7) is active, and it emits measuring pulses MI, cross feeding, cross reflection echo K resulting from distance sensor 41 is interpreted by distance sensor 45 as a direct reflection echo D, and an actually nonexistent pseudo-obstacle 8 is detected (in FIG. 7).
Similarly, as shown in FIG. 8, cross feeding, cross reflection echo K, which results from measuring pulse MI of distance sensor 41, may be interpreted by distance sensor 44 as a direct cross reflection echo K of distance sensor 45, which also leads to an erroneous detection of an actually nonexistent pseudo-obstacle 8.
FIG. 9 illustrates a limitation of measuring range R in connection with distance sensors 4 positioned at the front end of a motor vehicle. If the measuring range of the outer distance sensors, such as distance sensor 45, is limited with respect to the vehicle, cross feeding, cross reflection echos K having a longer running time may be excluded as being erroneous signals. However, the limitation of measuring range R has the disadvantage that obstacles are detected too late under certain circumstances.